Mirroring an object along Z-Axis results in improper lighting
I use Blender to create content for a game which makes use of a game engine which has a fundamental incompatibility issue with Blender. Where Blender defines the Z axis to be vertical, the game engine assigns th Vertical to the Y axis. It is worth checking to see if a similar incompatibility is involved here. Perhaps the render engine has reversed one axis (or more) so that where Blender interprets a positive value as meaning one direction, the render engine inerprets that same direction as a negative value.

I suspect that in your case if you orient your model so that the front of the car is at positive X, the Y axis in the render engine is oriented 180 degrees from the orientation in Blender. If this is the case, you may need to revise your workflow.